1. Field of the Invention
The present invention pertains to electron detection and electron detection devices, and, in particular, to an electron detector including an intimately-coupled scintillator-silicon photomultiplier combination that may be employed in an electron microscope and/or an X-ray detector, such as an X-ray detector used in an electron microscope.
2. Description of the Related Art
An electron microscope (EM) is a type of microscope that uses a particle beam of electrons to illuminate a specimen and produce a magnified image of the specimen. One common type of EM is known as a scanning electron microscope (SEM). An SEM creates images of a specimen by scanning it with a finely focused beam of electrons in a pattern across an area of the specimen, known as a raster pattern. The electrons interact with the atoms that make up the specimen, producing signals that contain information about the specimen's surface topography, composition, and other properties such as crystal orientation and electrical conductivity.
In a typical SEM, a beam of electrons is generated from an electron gun and accelerated to an anode 8 which is held at an accelerating voltage typically between 1 keV and 30 keV, although higher and lower voltage extremes are available on many instruments. The gun is positioned at the beginning of a series of focusing optics and deflection coils, called an electron column or simply “column” because its axis is typically vertical, which in turn is followed by a sample chamber or simply “chamber” housing the sample and accommodating a variety of detectors, probes and manipulators. Because electrons are readily absorbed in air, both the column and chamber are evacuated, although in some cases the sample chamber may be back-filled to a partial pressure of dry nitrogen or some other gas. The electrons may be initially generated by heating a filament, such as Tungsten or LaB6 (thermionic emission), by a strong electrical field (Cold Field Emission), or by a combination of the two (Schottky Emission). The electrons are then accelerated toward an anode, which is maintained at a high voltage called the “accelerating voltage”, then follow a path through the electron column, which contains a series of focusing lenses (usually electromagnetic) and scanning coils, such that a finely focused beam of electrons (on the order of 1-10 nanometers) is made to scan in a raster fashion as described above.
A special type of SEM dedicated to elemental analysis using X-rays is called an Electron Probe Micro-Analyzer (EPMA). By definition, the EPMA includes multiple wavelength-dispersive X-ray spectrometers, which employ the principle of X-ray diffraction to sort the X rays emitted from the sample according to their wavelength. Because wavelength spectrometers require a substantial amount of space inside the sample chamber and also require a precise beam-sample-spectrometer geometry, the sample in many EPMAs must be placed at a much longer distance (say 40 mm) from the final lens than in SEMs, sacrificing image resolution to accommodate the improved spectral resolution provided by the wavelength spectrometers.
The first implementation of the electron microscope was the Transmission Electron Microscope (TEM). In this case, an electron beam is generated in a fashion similar to that described for the SEM, and the beam is focused by similar lens arrangements. In the TEM, however, the sample resides within the objective lens field and the transmitted beam passes through one or more projection lenses (also, typically, electromagnetic). The most common TEM image is formed from the primary electrons that pass through the sample, which are influenced by absorption and diffraction. TEMs can provide image resolution on the order of 0.2 nm, several times better than CFE SEMs and more than an order of magnitude better than SEMs based on thermionic emission. Their disadvantages are their cost and the requirement for very small and very thin samples.
There are also instruments that combine some of the features of the SEM and TEM, called Scanning Transmission Electron Microscopes (STEM).
Another instrument which is frequently used in both development and failure analysis in the fields of semiconductor and nanotechnology implements both an electron beam and an ion beam integrated with a single sample chamber such that the electron beam can be used for normal SEM type imaging, while the focused ion beam (FIB) is used for high resolution milling of micro-regions of the sample, without requiring coarse repositioning of the sample. Such an instrument is called a Dual Beam or FIB/SEM. The milling is often used for creating and polishing cross sections in situ, allowing an SEM image of the cross section thus created to be obtained. In this instrument, gas injection systems may also be added, enabling the semiconductor or nano-materials designer, for example, to build or modify structures in situ, using a process conceptually similar to Chemical Vapor Deposition.
When the electron beam hits the specimen in all of these and similar instruments, some of the beam electrons (primary electrons) are reflected/ejected back out of the specimen by elastic scattering resulting from collisions between the primary electrons and the nuclei of the atoms of the specimen. These electrons are known as backscattered electrons (BSEs) and provide both atomic number and topographical information about the specimen. Some other primary electrons will undergo inelastic scattering causing secondary electrons (SEs) to be ejected from a region of the specimen very close to the surface, providing an image with detailed topographical information at the highest resolution. If the sample is sufficiently thin and the incident beam energy sufficiently high, some electrons will pass through the sample (transmitted electrons or TEs). Backscattered and secondary electrons are collected by one or more detectors which are respectively called a backscattered electron detector (BSED) and a secondary electron detector (SED), which each convert the electrons to an electrical signal used to generate images of the specimen. A transmitted electron detector (TED) can be of a similar type or can simply be a screen coated with a long-persistence phosphor.
Most electron detectors used to image BSEs and SEs employ a combination of a scintillator, a light guide (also called a “light pipe”) and a photomultiplier tube (PMT), as proposed by Everhart and Thornley (Everhart, T E and R F M Thornley (1960), “Wide-band detector for micro-microampere low-energy electron currents”, Journal of Scientific Instruments 37 (7): 246-248). A scintillator is a device made from a material that exhibits scintillation, which is the emission of photons, usually in the visible light, near UV or near IR regions of the spectrum, in response to radiation. An important requirement for scintillators used in scanning electron imaging is that they have a fast decay time, on the order of 100 nanoseconds or less, allowing the image to be recorded with high fidelity (without “smearing”) even when very rapid beam scanning rates are used (pixel dwell times on the order of 100 ns or less are available in modern SEMs). This is particularly important in automated image analysis, in which feature size, shape and position must be precisely calculated. Suitable scintillator materials include minerals such as YAG:Ce, YAP:Ce, ZnO:Ga, as well as some plastic scintillators. The scintillator in an electron detector thus generates light (photons) in response to electron impingement thereon. The light guide then collects some fraction of the generated light and transmits it outside the chamber or column to a PMT, which is a vacuum tube device, typically operated at 1000-1500 volts, that detects light. Thus, in such an electron detector, the scintillator emits photons caused by the impingement of BSEs or SEs and the PMT detects the presence of the fraction of photons that are successfully collected and transmitted to it by the light guide. In some references, the light guide, although its use is clearly indicated within detailed descriptions or figures in these references, is not called out specifically, but is assumed to be an integral part of the PMT or scintillator. The light guide and its attachment to the scintillator, however, play an important role in determining the device performance, and its required presence cannot be ignored.
In current electron microscopes, the predominant method of secondary electron detection is the Everhart Thornley (ET) detector just described. Backscattered electron detectors as well are often “ET type” detectors, i.e., the scintillator-light guide-PMT sequence is used, but the manner in which the light guide is attached to the scintillator differs significantly in the two applications, as further described below. The PMT of the BSED, the SED and the TED reside outside of the vacuum chamber of the EM because a PMT is a rather large, rigid device having a size on the order of several centimeters in all directions. As a result, each such electron detector is comprised of a scintillator positioned inside the vacuum chamber and a light pipe or similar light transporting device to carry a fraction of the generated photons out to a PMT residing outside the vacuum chamber through a chamber access port.
A typical SEM sample chamber will have a limited number of access ports available to accommodate accessory instruments or tools, such as one or more energy dispersive X-ray detectors, a wavelength dispersive X-ray spectrometer, an electron backscatter diffraction device, a cathodoluminescence spectrometer, micromanipulators, and, when laser or ion beam sources are available, a secondary ion mass spectrometer or a Raman spectrometer. These accessories, however, compete for port availability with the PMTs from the electron detectors, which may be several in number, including an SED, BSED, Low-Vacuum SED, and possibly a TED. If all the PMTs in an SEM could be eliminated, an equal number of ports would be made available for the mounting of additional important analytical tools that would otherwise be excluded from exploitation.
The SED and BSED differ as a result of the difference in the energy of the electrons detected. SEs are very low in energy, defined as 50 eV or less, and are drawn to the detector by a bias voltage of a few hundred volts and are then further accelerated to a scintillator biased with several thousand volts. The ability to influence the trajectory of SEs through a relatively low bias voltage enables the detector to be positioned to the side of the sample. BSEs have higher energy than SEs, with most BSEs having energy at or near the accelerating voltage of the primary beam, and their trajectory cannot be influenced by a voltage sufficiently low so as not to impact the primary beam. BSE imaging is therefore line-of-sight. Furthermore, when electron incidence is normal to the sample surface, which is typical, the distribution of BSEs in the space above the sample follows a cosine law, in which most of the electrons are backscattered along or near the axis of the electron column. These factors require that the BSED be placed immediately below the pole piece of the objective lens, in an annular fashion, such that the primary beam can pass through a small hole (e.g., 5 mm in diameter) in the center of the BSED. In order to achieve the highest image resolution in an SEM, samples must be placed as close as possible to the pole piece of the objective or “final” lens, i.e., samples should be imaged at the shortest possible “working distance”. A short working distance requires a short lens focal length, which in turn dictates a high lens current, minimizing aberrations and improving resolution. Therefore, the portion of the BSED which occupies space between the sample and the final lens pole piece, namely the scintillator and its attachment to the light pipe, must be as thin as possible. An important implication of this requirement in a scintillator-light guide combination is that the photons emitted from the scintillator can be collected only through the edge of the scintillator disc, as space above the scintillator cannot be sacrificed to accommodate the optics which would be required to redirect the photons into the light guide. The light guide in such assemblies is therefore coupled only to the periphery of the scintillator, through a C-shape coupling which grasps the disc by its thickness, not to its back surface (the surface opposite the surface on which the electrons impinge the scintillator). This results in a significant reduction in light collection efficiency for otherwise optimum BSED geometries. A further implication is that the light generated on the side of the scintillator disk opposite the light guide is not effectively transmitted to the PMT, meaning that there is always a topographical bias in the image (the view from one side of the column centerline dominates the image). In comparison, in ET detectors used as SEDs, the back-side of the scintillator disc is bonded directly to a mating surface of the light guide as there are no such geometrical and space restrictions.
A second type of BSED uses photodiodes also placed symmetrically around the centerline of the column, just under the pole piece of the final lens. The advantage of such a detector is that discrete photodiodes can be arranged around the column centerline, in “sectors”, and the signal collected from one or more of the sectors can be used to form the image. If sectors on one side only of the centerline are used to form the image, topographical contrast will dominate; if all of the sectors are used, topography will be eliminated and the image will be dominated by compositional contrast. In spite of this important capability, scintillator-based devices are often chosen over segmented photodiode detectors because they have much higher gain and can image at much faster scanning rates; in this case, the advantage of the selection of topographic or compositional modes provided by the photodiode detector is lost.
A third (and the least common) type of electron detector is a micro-channel plate (MCP) which, unlike the ET detectors, can be entirely contained inside the SEM, eliminating the need for a mounting port. The MCP, however, requires vacuum levels <10−6 torr, one or two orders of magnitude better than is typical inside the specimen chamber of the SEM. Furthermore, the MCP is slow and shares the disadvantage with the photodiode detector that it is unable to keep pace with the very short dwell times (high scanning rates) used in modern SEMs.
There is thus a great, unsolved need for an electron detector technology that is small, operates at low voltage, can be used inside the column or sample chamber, allows segmentation and high gain simultaneously, can be used in multiple, unique locations and indeed be positioned in situ via external manual or software control, and allows the number of access ports of an SEM that may be utilized for other analytical tools (and not electron detection) to be increased. There also accrue significant benefits in eliminating the requirement for a light guide, specifically the decreasing cost and complexity and increasing efficiency.